The present invention relates to an apparatus that controls tilting of an axle in an industrial vehicle such as a forklift. More particularly, the present invention pertains to an axle tilt control apparatus for locking an axle, which is tiltably supported by a vehicle's body frame, against tilting.
A typical industrial vehicle such as a forklift includes a rear axle that tilts relative to the body frame and a front axle that is fixed to the body frame. The rear axle tilts such that all the wheels always contact the ground even if the vehicle runs over bumps and dips on the road surface. This structure improves the stability of the vehicle.
However, when a load is on the forks, elevating the forks to a relatively high position elevates the center of gravity of the forklift. In this state, tilting of the rear axle inclines the body frame to right or to left and thus destabilizes the vehicle. Also, tilting of the rear axle destabilizes the vehicle when the forklift is traveling at a high speed. An apparatus for locking the rear axle has therefore been proposed. The apparatus monitors the stability of the vehicle based on the weight of the load on the forks, the height of the forks and the speed of the vehicle. When judging that the vehicle will be destabilized, the apparatus locks the rear axle to the body frame against tilting.
The forks are lifted and lowered relative to a mast supported by the body frame. The mast is tilted forward and rearward relative to the body frame. Even if the height of the forks and the weight of the load on the forks remain constant, the center of gravity of the vehicle is changed in accordance with the tilt angle of the mast. Specifically, the center of gravity is moved forward, or approaches the front axle, as the mast is tilted forward. The closer the center of gravity is to the fixed front axle, the more stable the vehicle is. Contrarily, the center of gravity is moved rearward, or approaches the tiltable rear axle, as the mast is leaned rearward. The closer the center of gravity is to the rear axle, the less stable the vehicle is. Therefore, the maximum height of the forks and the maximum weight of the carried load are increased by moving the center of gravity toward the front end of the vehicle. The maximum height and weight refer to maximum values of the height and weight that maintain the stability of the forklift at an acceptable level.
A typical prior art axle tilt control apparatus determines whether to lock the rear axle in accordance with the weight of the load on the forks and the height of the forks. Specifically, the rear axle is locked when the load's weight and the forks' height exceed predetermined determination values. The apparatus does not refer to the location of the forklift's center of gravity, which is moved in accordance with the tilt angle of the masts. In other words, if the center of gravity is changed due to tilting of the masts, the determination values, which determine when the axle is locked, remain unchanged.
As described above, the stability of the vehicle is lowest when the mast is most tilted rearward, that is, the center of gravity is at the most rearward position. Therefore, the determination values are determined when the mast is most tilted rearward. Specifically, the determination values are set to the maximum values of the load's weight and the forks' height when the masts are most tilted rearward. Thus, when the masts are not most tilted rearward, the rear axle may be locked even if the vehicle would be stable without locking the rear axle. In other words, the rear axle may be locked when it is not necessary. This is contrary to the goal of improving the stability of the vehicle by allowing the rear axle to tilt at the appropriate times.
When the rear axle is locked removing the load from the forks causes the rear axle to be unlocked. If the rear axle was locked while tilted, unlocking the rear axle causes the body frame to return to a horizontal position. At this time, the body frame swings to right and to left. As a result, the forks may collide with the bottom surface of the unloaded load.